A handheld flashlight may be powered by one or more electric storage batteries enclosed in a battery compartment inside the body of the flashlight. When the flashlight is turned on, electrical energy stored in the batteries is converted to light energy by a light emitter such as an incandescent bulb, light emitting diode (LED), or fluorescent bulb. The batteries, light emitter, and an electrical switch may be connected in a series electrical circuit. Some flashlights include an electrical circuit for dimming the light output from the flashlight or causing light to be emitted in a flashing pattern.
Some flashlights use high-capacity batteries such as lithium batteries, nickel metal hydride batteries, or other high-capacity battery technologies. High-capacity batteries may use a different battery casing to distinguish the batteries from zinc-carbon, alkaline, and rechargeable batteries.
A flashlight adapted to hold a lithium battery or other high-capacity battery may have a battery compartment that is the wrong internal diameter and possibly the wrong internal length for accepting another type of battery. If a battery of the correct size and shape is not available when the high-capacity battery needs to be replaced, the flashlight may be incapable of producing a useful amount of light until the correct replacement battery is located. It may be more difficult to obtain new high capacity batteries than carbon-zinc or alkaline batteries in remote areas, after normal business hours, or during an emergency.